1. Field of the Invention
The present invention relates to a secondary battery comprising an electrolyte, a positive electrode and a negative electrode, and an electrolyte used therefor.
2. Description of the Related Art
In recent years, many small portable electric equipments, such as video cameras combined with video tape recorder, cellular phones, and laptop computers, appear, and the miniaturization and the weight saving thereof are investigated. Batteries with a high energy density are required as a portable power supply for these electric equipments. This advances the research and development for improving the energy density of the batteries, especially of secondary batteries, actively. Specifically, lithium ion secondary batteries, where a carbon material and LiCoO2 or the like are used respectively as a negative electrode and for a positive electrode, have a high energy density compared with conventional aqueous electrolyte secondary batteries such as a lead battery, a nickel-cadmium battery, and a nickel-hydrogen battery. Therefore, the lithium ion secondary batteries are widely used as an optimal power supply of portable electric equipments such as notebook personal computers (hereinafter referred as notebook computers), cellular phones and personal digital assistances (PDA).
Particularly, notebook computers have a hard disk drive as a motor drive, and a liquid crystal display using luminescence, and the speed of a central processing unit (CPU) thereof is higher, so power consumption thereof becomes much higher. Electric equipments requiring high power such as the notebook computers use plural battery packs which are connected in series and in parallel. Micro porous films which are made of a polyolefin such as polyethylene and polypropylene are generally used as a separator in the batteries using nonaqueous electrolyte such as the lithium ion batteries. Some of these micro porous films exhibit shutdown functions of closing pores of the micro porous film when a battery internal temperature increases unusually.
However, in the above electric equipments, electric power is consumed not only by operations of each device inside the electric equipments, but also by the attendant heat generation, and the heat generation always increases an internal temperature of the electric equipments to high temperatures in the order of 50° C. to 70° C. in use. That is, in a type of the built-in battery equipments, exposing the battery to high temperature atmospheres causes capacity deterioration and charge and discharge in the high temperature atmospheres causes also the capacity deterioration. This reduces discharge capacities thereof in real operating conditions and cannot provide time enough to drive the equipments. Further, heat deteriorates the polyethylene micro porous films which are used as the separator of the lithium batteries and the like.